编译器错误变量“未捕获"；线程池的内部lambda函数 [英] Compiler Error variable "Not captured" inside lambda function for threadpool

问题描述

我正在学习c ++中的多线程，并试图建立线程池，但是遇到了一个编译器错误，提示错误:未捕获'mapperNodes'"和错误:未捕获'命令'".我已经读过一些有关使用"this"捕获lambda中的变量的信息，但到目前为止没有任何工作.

I'm learning about multithreading in c++ and am trying to set up a thread pool, but am getting a complier error saying "error: ‘mapperNodes’ is not captured" and "error: ‘command’ is not captured". I've read a bit about using "this" to capture the variables in the lambda, but so far nothing has been working.

如何在下面的代码的线程池lambda函数中使用command和mapperNoders变量?

How can I use the command and mapperNoders variables in the thread pool lambda function in the code below?

void MapReduceServer::spawnMappers() throw() {
  vector<string> mapperNodes(nodes);
  random_shuffle(mapperNodes.begin(), mapperNodes.end());
  string command = buildCommand(mapperNodes[0], executablePath, mapperExecutable, mapOutputPath);

  ThreadPool pool(numMappers);//numMappers = 8

  for (size_t id = 0; id < numMappers; id++) {
    pool.schedule([id] {
      cout << oslock << "Thread (ID: " << id << ") has started." << endl << osunlock;

      spawnWorker(mapperNodes[0], command); /*compiler error here*/

      cout << oslock << "Thread (ID: " << id << ") has finished." << endl << osunlock;
  });
}

推荐答案

编写pool.schedule([id]{ /*...*/ });时，您告诉编译器您的lambda只需要副本的值id变量，仅此而已.

When you wrote pool.schedule([id]{ /*...*/ });, you told the compiler that your lambda only wants a copy of the value of the id variable, and nothing else.

要将所有MapReduceServer::spawnMappers()变量(的副本)提供给lambda，可以将[id]更改为[=].

To make (a copy of) all variables of MapReduceServer::spawnMappers() available to the lambda, you can change [id] for [=].

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